In a typical industrial production comprising automated processes, such as petrochemical, chemical or pharmaceutical processing systems, pulp and paper production systems or power plants, a process control system is used in order to monitor various process variables and control the processes accordingly. For example, a process variable such as the fluid level of a tank or the temperature of a fluid in a process step may give rise to a certain control action, for example opening of a valve. Different sensor devices and actuators are used for this end and sensor data is transmitted in hardwired communication networks to a controller regulating the processes. Several such controllers may be interconnected and also connected to supervisory control work stations wherein users or operators may supervise and control the processes.
The above hardwired process control system design involves installing communication cables, which is labour intensive and costly. Further, should there arise a need for installing additional sensor devices, e.g. if a new process step is introduced, more cumbersome and expensive communication cable installation is required, as in case of a need to replace existing communication cables. Further yet, the communication wiring is expensive also in terms of material and may degrade over time.
An alternative to the above described process control systems could therefore be to introduce wireless control in the process industry. The introduction of a wireless process control system requires careful consideration. One issue is the reliability of data transfer, and another the battery power supply of the wireless nodes of the process control system. There may be a trade-off between these two issues in that the data transfer reliability in some instances requires many messages to be sent, which result in a high traffic load and thus shortened battery life time of the wireless devices.
As a particular example it can be mentioned that several parts of the wireless process control system are typically not synchronized. Many tasks need execution in order for sensor data to reach the intended control application and several of these tasks are not synchronized. Known solutions comprise the use of equidistant sampling and periodical transmission of the information from the sensor device to the process control system. Delays can be compensated for by the control application using time stamp information. A drawback is the frequent synchronization messages required by such accurate time stamps, which increases the traffic load and depletes the battery. Alternatively expensive clock circuitry could be used in the wireless sensor devices, but this would increase the overall costs of the process control system. Further, equidistant sampling results in high traffic load during steady state when the periodic transmissions are performed even though no control action is needed.